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Trade receivables - Trade receivables from closely monitored countries (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of geographical areas [line items]        
Gross trade receivables $ 1,429 $ 1,438    
Provisions for doubtful trade receivables (68) (48) $ (54) $ (77)
Greece, Italy, Portugal, Spain, Brazil, Russia, Turkey, Saudi Arabia, and Argentina [member]        
Disclosure of geographical areas [line items]        
Provisions for doubtful trade receivables (15) (13)    
Not overdue [member]        
Disclosure of geographical areas [line items]        
Gross trade receivables 1,137 1,135    
Provisions for doubtful trade receivables (2) (1)    
Not overdue [member] | Greece, Italy, Portugal, Spain, Brazil, Russia, Turkey, Saudi Arabia, and Argentina [member]        
Disclosure of geographical areas [line items]        
Gross trade receivables 211 209    
Past due for more than one year [member]        
Disclosure of geographical areas [line items]        
Gross trade receivables 49 36    
Provisions for doubtful trade receivables (43) (31)    
Past due for more than one year [member] | Greece, Italy, Portugal, Spain, Brazil, Russia, Turkey, Saudi Arabia, and Argentina [member]        
Disclosure of geographical areas [line items]        
Gross trade receivables $ 14 $ 10